1. Field of the Invention
The present invention relates to an exhaust gas control valve used for an exhaust muffler of a motor vehicle or the like.
2. Description of the Related Art
An exhaust gas control valve is ordinarily arranged in an exhaust muffler on an opening edge portion of an exhaust pipe or on a communicating hole portion of a partition plate, such as a baffle plate, of the muffler so as to suppress fall in engine output power due to exhaust gas pressure loss. The control valve opens and closes according to exhaust gas pressure so that it increases the effect on attenuation of exhaust noise by shifting exhaust gas passages connectable with plural small chambers defined in the inside of the muffler in an optimum combination of the chambers according to exhaust gas pressure fluctuation.
A conventional exhaust gas control valve of this kind is disclosed in Japanese patent laying-open publication No. 2002-235536. This conventional valve has a valve seat provided on an opening edge portion of an exhaust pipe arranged in a muffler main body, a valve plate rotatable around a shaft fixed on a projecting portion of the valve seat and contactable with an annular seat surface of the valve seat for closing the valve, and a coil spring for urging the valve plate toward its closed position.
The control valve is closed by spring force of the coil spring during low engine speed operation, where exhaust gas pressure is lower than the spring force. This closing of the control valve provides the muffler with plural small chambers so that one of the small chambers acts as a resonant chamber for decreasing exhaust noise prior to prevention against fall of engine output power. During high engine speed operation, the control valve is opened by high exhaust gas pressure stronger than the spring force of the spring, shifting its exhaust gas passages to form an enlarged chamber by connecting the small chambers for suppressing the fall of the engine output power prior to the exhaust noise reduction.
This conventional exhaust gas control valve, however, encounters the following problems. The spring force generated by the coil spring becomes larger with an opening amount of the valve, urging the valve plate strongly toward the valve seat during the high engine speed operation. Consequently, the valve is held down in its opening amount during the high engine speed operation, not discharging the exhaust gas in the muffler into the air sufficiently. This causes back pressure in the muffler to rise, lessening the engine output power.
On the other hand, the spring may be set to have spring force lower, urging the valve plate toward the valve seat weakly in order to decrease the back pressure during the high engine speed operation. Consequently, the valve is easily opened in relatively low engine speed operation and its opening amount becomes larger than expected. This premature opening of the valve provides the expanded chamber in the muffler by shifting its exhaust gas passages, causing insufficient reduction in the exhaust gas noise during the low engine speed operation.
That is, the conventional control valve has a trade-off relationship between the exhaust gas noise reduction during the low engine speed operation and the back pressure reduction during the high engine speed operation, not obtaining them at the same time.
It is, therefore, an object of the present invention to provide an exhaust gas control valve which overcomes the foregoing drawbacks and can decrease exhaust gas noise in a low engine speed operation and back pressure in a muffler in a high engine speed operation at the same time.